Protozoan parasites are a serious health threat in many areas of the world. Trypanosoma cruzi (T. cruzi) is one such parasite that infects millions of individuals, primarily in Central and South America. Infections with this parasite can cause Chagas' disease, which can result in chronic heart disease and a variety of immune system disorders. It is estimated that 18 million people in Latin America are infected with T. cruzi, but there is no definitive treatment for the infection or its clinical manifestations.
The most significant route of transmission in areas where the disease is endemic is through contact with an infected triatomid bug. In other areas, however, blood transfusions are the dominant means of transmission. To inhibit the transmission of T. cruzi in such regions, it is necessary to develop accurate methods for diagnosing T. cruzi infection in individuals and for screening blood supplies. Blood bank screening is particularly important in South America, where 0.1%-62% of samples may be infected and where the parasite is frequently transmitted by blood transfusion. There is also increasing concern that the blood supply in certain U.S. cities may be contaminated with T. cruzi parasites.
The diagnosis of T. cruzi infection has been problematic, since accurate methods for detecting the parasite that are suitable for routine use have been unavailable. During the acute phase of infection, which may last for decades, the infection may remain quiescent and the host may be asymptomatic. As a result, serological tests for T. cruzi infection are the most reliable and the most commonly used.
Such diagnoses are complicated, however, by the complex life cycle of the parasite and the diverse immune responses of the host. The parasite passes through an epimastigote stage in the insect vector and two main stages in the mammalian host. One host stage is present in blood (the trypomastigote stage) and a second stage is intracellular (the amastigote stage). The multiple stages result in a diversity of antigens presented by the parasite during infection. In addition, immune responses to protozoan infection are complex, involving both humoral and cell-mediated responses to the array of parasite antigens.
While detecting antibodies against parasite antigens is the most common and reliable method of diagnosing clinical and subclinical infections, current tests are expensive and difficult. Most serological tests use whole or lysed T. cruzi and require positive results on two of three tests, including complement fixation, indirect immunofluorescence, passive agglutination or ELISA, to accurately detect T. cruzi infection. The cost and difficulty of such tests has prevented the screening of blood or sera in many endemic areas.
An improved method of detecting T. cruzi infection was disclosed in U.S. Pat. No. 5,304,371, which is incorporated herein by reference. In that patent, an antigenic epitope of T. cruzi was disclosed that detected antibodies to T. cruzi, and thus infection with the parasite, in most infected patients. However, while this method is an improvement over prior methods, the sensitivity of the technique is only about 93% (i.e., only about 93% of infections could be diagnosed).
Similar difficulties arise in the diagnosis of Leishmania infections. A variety of species of Leishmania infect humans, causing human diseases characterized by visceral, cutaneous, or mucosal lesions. Millions of cases of leishmaniasis exist worldwide, and at least 400,000 new cases of visceral leishmaniasis (VL) are diagnosed annually. Leishmania species are transmitted to humans and other mammals by the bite of a sand fly or through blood transfusions with contaminated blood.
VL is generally caused by L. donovani in Africa and India, L. infantium in Southern Europe, or L. chagasi in Latin America. In VL, high levels of parasite specific antibodies are observed prior to the detection of antigen specific T cell responses (Ghose et al., Clin. Exp. Immunol. 40:318-326, 1980). This antibody response has been used for serodiagnosis (commonly by immunofluorescence techniques) of infection with L. chagasi and L. donovani. Those serodiagnosis methods currently available for diagnosing VL typically use whole or lysed parasites. Such methods are prone to inaccuracy and cross-reaction with a variety of other diseases, and fail to detect some cases of the potentially fatal disease early enough to allow effective treatment.
Accordingly, there is a need in the art for more specific and sensitive methods of detecting T. cruzi and Leishmania infections in blood supplies and individuals. The present invention fulfills these needs and further provides other related advantages.